1. Field of the Invention
The present invention relates to ink jet printing apparatuses and methods, and in particular to configurations for reducing image defects caused by errors in the conveyed distance of a print medium such as printing paper.
2. Description of the Related Art
It is known that in ink jet printing apparatuses black and white stripe-like density unevenness occurs in printed images, caused by errors in the conveyed distance. The conveyance mechanisms of ink jet printing apparatuses are generally composed of parts such as conveyance rollers, pinch rollers, sheet discharge rollers and spurs. These parts have variation in their dimensions. The dimensional variation of each of the parts of this kind of conveyance mechanism causes a decline in precision in conveying the print medium, and causes the occurrence of the above mentioned error in the conveyed distance. In addition to the above mentioned decline in precision in conveying the print medium, it is also thought that flicking of the printing paper, that is, the existence of a kicking phenomenon when the tail end of the printing paper breaks free from the nip of the paper feed and pinch rollers, is also a cause of conveyance distance errors.
FIG. 1 is a diagram that schematically illustrates the density unevenness that occurs due to conveyance distance error, caused by the above mentioned kicking phenomenon. In FIG. 1, 301 denotes a white stripe-like density unevenness, which is a belt shaped region of a fixed width and low density, caused by kicking. Kicking is caused not only by the mechanical elements of the printing apparatus, but also by a variety of factors such as the unevenness among different types of print media and differences between print medium lots. As a result there is unevenness in whether kicking occurs or does not occur, and in the cases where kicking occurs there is also an unevenness in the distance of the conveyance error. Accordingly, in general, it is very difficult to devise countermeasures by way of controlling conveyance, such as by taking into account such positional shifts in advance and correcting the conveyance at the location where the kicking occurs, according to each of the various factors.
FIGS. 2A and 2B are diagrams that explain the image defects when the above mentioned kicking occurs. The printing example shown in these figures illustrates a case of so-called multi-pass printing wherein print data is allocated (partitioned) into multiple scans and printing is performed by carrying out multiple scans of the print head at the same image area (in the present example 2 pass printing is carried out by partitioning the data into 2 scans). In FIGS. 2A and 2B the numbers written on each dot indicate which pass of the 2 pass printing by which the dot is printed. More particularly, in the examples illustrated in FIGS. 2A and 2B dots arranged in a staggered pattern are printed in the first pass and in the second pass dots are printed at locations that fill the gaps of the dots arranged in the staggered pattern of the first pass.
FIG. 2A schematically illustrates the case where the dots are ideally arranged, that is, where there is no kicking and as a result there are no conveyance distance errors. In contrast, FIG. 2B illustrates a dot pattern for the case where kicking occurs at the print medium conveyance between the first pass and the second pass and because of this there is a conveyance distance error, in the direction of conveyance. As shown in FIG. 2B the dots of the second pass are printed at locations that are shifted, in the print medium conveyance direction, from the standard position of second pass dots in relation to first pass dots. As a result, the coverage of the surface of the print medium by dots is decreased in comparison to the example illustrated in FIG. 2A. Because of the decrease in coverage at the macroscopic level there is a decrease in density, at the area where printing is completed by the first pass and second pass, and a white stripe-like density unevenness 301 occurs as shown in FIG. 1. This type of density unevenness occurs not only as the result of comparatively large conveyance distance errors such as from kicking but also as the result of comparatively small conveyance distance errors resulting from imprecision in the conveyance mechanism as mentioned above.
It is possible to reduce the above described density unevenness, caused by dot position shifts in the conveyance direction of the print medium, by devising a mask pattern for use in multi-pass print data partitioning.
As for the staggered pattern and the complementary pattern that fills the gaps of the staggered pattern, illustrated in FIGS. 2A and 2B, because the arrangement of mask pixels that permit printing are ordered or periodic patterns, the area factor easily varies in relation to conveyance distance errors such as kicking. In contrast, as shown in FIG. 3, because the mask pixels that authorize printing (the pixels that are shown in black in FIG. 3 are hereafter also referred to as “print permitted pixels”) are arranged randomly, as in the case of a so-called random mask pattern, a coverage decrease caused by a conveyance distance error is unlikely to occur. FIGS. 4A and 4B are diagrams that explain this and are similar to FIGS. 2A and 2B. FIG. 4A illustrates a printing state with no errors in the distance conveyed, in the case where a random mask pattern such as that of FIG. 3 is used and partitioning of print data into first and second passes is performed. As can also be understood from this figure the arrangement of each of the dots printed in the first pass and the second pass and the arrangement of the first pass and the second pass with respect to each other are random. In contrast FIG. 4B schematically illustrates a printing state where a conveyance distance error such as kicking has occurred. As can be understood by comparing FIGS. 2B and 4B, the decrease in coverage, due to conveyance distance error, is smaller in the example illustrated in FIG. 4B. The reason for this is because, as compared to the case of a staggered pattern mask, where print permitted pixels in the pattern of the same pass are not arranged consecutively in the direction of conveyance, in the case of a random mask portions are included where print permitted pixels of the pattern of the same pass are arranged consecutively. That is, in the case where in the same pass dots are printed having a consecutive arrangement in the direction of conveyance, even if there is the occurrence of a print position shift in the conveyance direction between passes, because consecutively arranged dots are shifted together the area where other portions of the print medium are visible as a result of the shift is decreased. In other words, by making use of a random mask pattern the decrease in dot coverage rate caused by errors in the distance conveyed is decreased.
It should be noted that, taking X as the direction of print head scanning and Y as the direction of print medium conveyance, because the conveyance distance error is a shift in the Y direction, the influence of conveyance distance error is decreased as mask pattern dot arrangement continuity is provided in the Y direction. However, if continuity is provided only in the Y direction influence of variation in the X direction (for example, a print location shift caused by a dot print position shift in the forward or return direction of a bi-directional print, or caused by a change in the carriage velocity) is increased. For this reason it is preferable to make use of a mask pattern that has a white noise characteristic without a peak spatial frequency characteristic in a specific direction.
The use of a random mask in connection with multi-pass print data partitioning is disclosed in Japanese Patent Laid-Open No. H07-052390 (1995). As above, a mask pattern, such as a random mask pattern wherein the arrangement of the dots printed thereby includes a comparatively large number of low frequency components, is preferably used in the reduction of density unevenness, such as stripes, caused by a print position shift in the direction of conveyance.
However, although mask patterns such as those above, wherein the number of low frequency components of the dot arrangement is comparatively large, are effective against print position shifts, in the conveyance direction, that occur between passes, uneven distribution of overlapping dots caused by a print position shift occurs easily and macroscopically these overlapping dots cause image graininess. That is, orderly mask patterns such as the staggered mask pattern mentioned above do not, because of a print position shift, produce dot overlap having an ordered arrangement or cause image graininess. In contrast to this, in randomized patterns, wherein the dot arrangement has a large number of low frequency components, the arrangement of overlapping dots, due to a print position shifts between passes, is unevenly distributed. In other words, the dot overlap is not satisfactorily dispersed. Accordingly, these unevenly distributed overlapping dots bring about graininess in the printed image.
FIG. 5 is a diagram that explains this aspect. 501 to 504 of FIG. 5 illustrate the changing position of the print head relative to the printing paper, while the printing paper is conveyed by the conveying unit. The same figure also illustrates an example of 2 pass printing where printing is carried out via 2 scans over the same fixed print region. Furthermore, FIG. 5 illustrates an example where kicking occurs in the conveyance of the pass interval when moving from print head position 502 to print head position 503 and where in the other pass intervals there are no distance errors in conveying the printing paper. When there is the occurrence of a conveyance distance error, such as kicking, in conveying the print medium from print head position 502 to print head position 503 a decrease in image quality such as that of printed image 508 occurs. In greater detail, because printing of the image region denoted by the arrow 505 is carried out when the print head is at positions 501 and 502 there is no influence of kicking and a decrease in image quality does not occur. Because printing of the image region denoted by the arrow 506 is carried out when the print head is at positions 502 and 503 the influence of the kicking that occurs at the conveyance of the printing paper to position 503 is felt and image graininess worsens. Furthermore, as printing of the image region denoted by the arrow 507 is carried out when the print head is at positions 503 and 504, printing is carried out after the occurrence of the kicking, and because there is no occurrence of a conveyance distance error in the conveyance between position 503 and position 504 worsening of the above mentioned graininess does not occur. In this manner when viewing the entire image 508, graininess differs only in region 506, and image quality is decreased.
As described above, the manner in which conveyance distance errors exert influence differs according to the mask pattern that is used and there is a problem wherein the deterioration of image quality become remarkable in the case where a non-suitable mask pattern is used. Also, it has been known that the above mentioned graininess becomes more perceptible to the human eye as the volume of the ink drops ejected from the print head increase, that is, as the size of the dots printed by the ink drops increases. Furthermore, it is also known that density of the formed dots increases as the color density of the ink dyes, etc. increases, and in the same manner the graininess is more easily perceived.